Apparatus for cutting substrate and method using the same

ABSTRACT

An apparatus for cutting a substrate includes a laser oscillator generating a femtosecond laser beam, a first beam splitter splitting the femtosecond laser beam into first and second femtosecond laser beams, a first condenser lens receiving the first femtosecond laser beam and condensing the first femtosecond laser beam to have a first focal depth, a second condenser lens receiving the second femtosecond laser beam, and condensing the second femtosecond laser beam to have a second focal depth different from the first focal depth, and a second beam splitter receiving and splitting the first femtosecond laser beam condensed through the first condenser lens and the second femtosecond laser beam condensed through the second condenser lens, and irradiating the split first and second femtosecond laser beams at different positions on a substrate to be cut.

This application claims the benefit of Korean Patent Application No.P2004-89703, filed in Korea on Nov. 5, 2004, which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) device,and more particularly, to an apparatus for cutting a substrate and amethod using the same. Although the present invention is suitable for awide scope of application, it is particularly suitable for improvingyield in manufacturing liquid crystal display devices.

2. Discussion of the Related Art

With the recent rapid development of information communication fields,the industries associated with displays adapted to display desiredinformation are gaining importance. Of such information displays,cathode ray tubes (CRTs) have continuously drawn attention by virtue ofadvantages such as reproducibility of diverse colors and superior screenbrightness. Due to the recent demand for large-size, portable andhigh-resolution displays, however, the development of flat paneldisplays is in high demand, in order to replace CRTs which are heavy andbulky.

Flat panel displays are applicable to wide and diverse fields such ascomputer monitors and monitors for both aircraft and spacecraft. Ascurrently-developed or commercially-available flat panel displays, thereare LCDs, electro-luminescent displays (ELDs), field emission displays(FEDs), plasma display panels (PDPs), and the like.

A process of manufacturing such flat panel displays usually involvesseparating a fragile mother substrate into a plurality of unit LCDsusing a cutting process. On the mother substrate, a plurality of unitelements, such as semiconductor chips are formed in a matrix form tocreate large-scale integrated circuits.

There are basically two cutting process used for separating the fragilemother substrate, which may be made of glass, silicon, or ceramic. Thefirst process is a dicing method in which cutting grooves are formed onthe substrate using a diamond blade having a thickness of 50 to 200 μmwhile the diamond is rotated at high speed. The second process is ascribing method in which cutting grooves are formed on a surface of thesubstrate by a scribing wheel made of a diamond having a thickness of0.6 to 2 mm, so as to form a crack in a thickness direction of thesubstrate.

The dicing method is suitable for cutting of a substrate formed with athin film or a convex portion at a surface of the substrate because avery thin blade is used, as compared to the scribing method. In thedicing method, however, frictional heat is generated at a region wherethe blade performs a cutting process. Furthermore, since the cuttingprocess is carried out under the condition in which cooling water issupplied to the cutting region, the dicing method is not considered tobe a method suitable for a flat panel display which includes metalportions, such as metal electrode layers or metal terminals.

In other words, in the dicing method, it is difficult to completelyremove the cooling water after the cutting process. When moistureremains due to the incomplete removal of the cooling water, there may bea possibility that the metal portions of the flat panel display may beeroded. Furthermore, the dicing method has a problem of a prolongedcutting time, thereby lowering yield, as compared to the scribingmethod.

On the other hand, it is unnecessary to use cooling water in thescribing method. Accordingly, the scribing method exhibits superiorthroughput, as compared to the dicing method. Also, since the cuttingtime required in the scribing method is shorter than that of the dicingmethod, the scribing method has an advantage in better yield.

FIG. 1 is a cross-sectional view illustrating the related art LCDdevice. This LCD device is manufactured in accordance with the followingmethod. For simplicity, descriptions will be made only in conjunctionwith one pixel region.

As shown in FIG. 1, a gate electrode 11 of a conductive material, suchas metal, is deposited on a first transparent substrate 10 at apredetermined region. A gate insulating film 12 of a silicon nitride(SiNx) or silicon oxide (SiO₂) is then deposited over the entire uppersurface of the first substrate 10 including the gate electrode 11.

Thereafter, an active layer 13 of amorphous silicon is formed on thegate insulating film 12 at a region corresponding to the gate electrode11. An ohmic contact layer 14 is formed on the active layer 13 atregions corresponding to respective lateral edge portions of the activelayer 13. The ohmic contact layer 14 is formed of doped amorphoussilicon.

Source and drain electrodes 15 and 16, which are formed of a conductivematerial such as metal, are subsequently formed on the ohmic contactlayer 14. The source and drain electrodes 15 and 16 constitute a thinfilm transistor T, together with the gate electrode 11. Meanwhile,although not shown in the drawing, the gate electrode 11 is connected toa gate line, and the source electrode 15 is connected to a data line.The gate line and the data line cross each other, and define a pixelregion.

A protective film 17 is then formed over the entire upper surface of thefirst substrate 10 including the source and drain electrodes 15 and 16.The protective film 17 is formed of a silicon nitride, silicon oxide, ororganic insulating material. The protective 17 has a contact hole 18through which a predetermined portion of the surface of the drainelectrode 16 is exposed. Thereafter, a pixel electrode 19 of atransparent conductive material is formed on the protective film 17 atthe pixel region. The pixel electrode 19 is connected to the drainelectrode 16 via the contact hole 18.

A first orientation film 20 is then formed over the entire upper surfaceof the first substrate 10 including the pixel electrode 19. The firstorientation film 20 is polyimide, and has a surface on which themolecules of the first orientation film 20 are oriented in apredetermined direction. Meanwhile, a second transparent substrate 31 isarranged over the first substrate 10 while being vertically spaced apartfrom the first substrate 10 by a predetermined distance.

A black matrix 32 is formed on a lower surface of the second substrate31 at a region corresponding to the thin film transistor T of the firstsubstrate 10. Although not shown in the drawing, the black matrix 32also covers a region except for the pixel electrode 19.

A color filter 33 is then formed on the second substrate 31 beneath theblack matrix 32. Practically, color filters are arranged in the form ofrepeated filter patterns of red (R), green (G), and blue (B), each ofwhich corresponds to one pixel region.

A common electrode 34 of a transparent conductive material issubsequently formed on the second substrate 31 beneath the color filter33. A second orientation film 35 is then formed on the second substrate31 beneath the common electrode 34. The second orientation film 35 is ofpolyimide, and has a surface on which the molecules of the secondorientation film 35 are oriented in a predetermined direction. Then, aliquid crystal layer 40 is formed between the first orientation film 20and the second orientation film 35.

The above-described LCD device is manufactured using an array substratefabrication process involving formation of thin film transistors andpixel electrodes on a substrate to fabricate an array substrate, a colorfilter substrate fabrication process involving formation of colorfilters and a common electrode on another substrate to fabricate a colorfilter substrate, and a liquid crystal panel fabrication processinvolving arrangement of the fabricated substrates, injection andsealing of a liquid crystal material, and attaching polarizing plates,to thereby complete in fabricating a liquid crystal panel.

FIG. 2 is a flow chart illustrating the related art LCD manufacturingmethod.

In accordance with this method, a thin film transistor (TFT) arraysubstrate including TFTs, and a color filter substrate including colorfilters are first prepared (S1), as shown in FIG. 2. The TFT arraysubstrate is fabricated by repeatedly performing processes of depositinga thin film and pattering the deposited thin films. In this case, thenumber of masks used for patterning of thin films in the fabrication ofthe TFT array substrate represents the number of processes used in thefabrication of the TFT array substrate. Currently, research is beingactively made to reduce the number of masks, thereby reducing themanufacturing cost.

The color filter substrate is fabricated by sequentially forming a blackmatrix for preventing light leakage through a region except for pixelregions, such as R, G, and B color filters and a common electrode. Thecolor filters may be formed using one of a dyeing method, a printingmethod, a pigment dispersion method, an electro-deposition method, orthe like. Currently, the pigment dispersion method is mostly used.

Thereafter, an orientation film is formed over each substrate todetermine an initial alignment direction of liquid crystal molecules(S2). The formation of the orientation film is achieved using a processfor coating a polymer thin film, and treating the surface of the polymerthin film such that the molecules of the polymer thin film on thetreated surface are oriented in a predetermined direction. Generally,polyimide-based organic materials are mainly used for the orientationfilm. For the orientation method, a rubbing method is mostly used.

In accordance with the rubbing method, the orientation film is rubbed ina predetermined direction, using a rubbing cloth. This rubbing method issuitable for mass production because treatment for orientation can beeasily achieved. Also, the rubbing method has advantages of stableorientation and easy control of a pre-tilt angle. Recently, an opticalorientation method has been developed and practically used that achievesorientation using polarized beams.

Next, a seal pattern is formed at one of the two substrates (S3). Theseal pattern is arranged around a region where an image is displayed.The seal pattern has a port for injecting a liquid crystal material, andserves to prevent the injected liquid crystal material from leaking.

The seal pattern is formed by forming a thermosetting resin layer tohave a predetermined pattern. For the formation of the seal pattern, ascreen printing method using a screen mask, and a seal dispenser methodusing a dispenser may be used.

Currently, the screen printing method is mainly used because it has amore convenient process. However, the screen printing method also has adrawback in that products with poor quality may be produced because thescreen mask may come into contact with the orientation film.Furthermore, the screen mask cannot easily cope with a large-sizedsubstrate size. For this reason, substitution of the seal dispensermethod for the screen printing method is being gradually increased.

Subsequently, spacers having a predetermined size are sprayed on one ofthe TFT array substrate and the color filter substrate to maintain anaccurate and uniform space between the two substrates (S4). For a methodof spraying spacers, there are a wet spray method in which spacers aresprayed in a state of being mixed with alcohol, and a dry spray methodin which just spacers are sprayed alone. For the dry spray method, thereis an electrostatic spray method using static electricity and an ionicspray method using pressurized gas. Since LCDs are vulnerable to staticelectricity, the ionic spray method is mainly used.

Thereafter, the two substrates of the LCD (i.e., the TFT array substrateand the color filter substrate) are arranged such that the seal patternis interposed between the substrates. In this state, the seal pattern iscured under pressure to attach the substrates (S5). In this case, theorientation films of the substrates face each other, and the pixelelectrodes and the color filters correspond to each other one by one.

Next, the joined substrates are cut to be separated into a plurality ofunit liquid crystal panels (S6). Generally, a plurality of liquidcrystal panels, each of which will be one LCD device, are formed on onesubstrate sheet, and are then separated into individual ones, to achievean enhancement in manufacturing efficiency and a reduction inmanufacturing costs.

The liquid crystal panel cutting process includes a scribing process forforming a crack in the surface of each substrate using a scribing wheelmade of a diamond material having hardness higher than that of thesubstrate, which is made of, for example, glass. The liquid crystalpanel cutting process further includes a breaking process forpositioning a breaking bar at a portion of the substrate where the crackis formed. Subsequently, a predetermined pressure is applied to thebreaking bar, thereby cutting the substrate in a direction along whichthe crack extends.

Next, a liquid crystal material is injected between the two substratesof each liquid crystal panel (S7). For the injection of the liquidcrystal, a vacuum injection method is mainly used which utilizes apressure difference between the interior and exterior of the liquidcrystal panel. Micro air bubbles may be present amongst the liquidcrystal molecules injected into the interior of the liquid crystalpanel, so that bubbles may be present in the interior of the liquidcrystal panel, thereby causing the liquid crystal panel to have poorquality. In order to prevent such a problem, accordingly, it isnecessary to perform a de-bubbling process in which the liquid crystalis maintained in a vacuum state for a prolonged time to remove bubbles.

After completion of the liquid crystal injection, the injection port issealed to prevent the liquid crystal from leaking out of the injectionport. The sealing of the injection port is achieved by coating anultraviolet-setting resin over the injection port, and irradiatingultraviolet rays to the coated resin, thereby setting the coated resin.

Next, polarizing plates are attached to the outer surfaces of the liquidcrystal panel fabricated in the above-mentioned manner, and drivingcircuits are then connected to the liquid crystal panel. Thus, thefabrication of an LCD device is complete (S8). Hereinafter, a relatedart substrate cutting apparatus and a related art substrate cuttingmethod using the same will be described with reference to the annexeddrawings.

FIG. 3 is a schematic view illustrating a related art scribing device.As shown in FIG. 3, the related art scribing device includes a table 51,on which a substrate G is positioned, a vacuum chucking unit (not shown)adapted to fix the substrate G to the table 51, and a pair of parallelguide rails 52 for pivotally supporting the table 51 in a suspendedstate while allowing the table 51 to be movable in a Y-axis direction.The scribing device also includes a ball screw 53 for moving the table51 along the guide rails 52, a guide bar 54 installed above the table 51such that the guide bar 54 extends in an X-axis direction, and ascribing head 55 mounted on the guide bar 54 such that the scribing head55 can slide in the X-axis direction along the guide bar 54. Thescribing device further includes a motor 56 for sliding the scribinghead 55, a tip holder 57 mounted to a lower end of the scribing head 55such that the tip holder 57 is vertically movable while being rotatable,and a scribing wheel 1 rotatably mounted to a lower end of the tipholder 57.

In the related art substrate cutting method using the above-mentionedscribing device, a crack having a certain depth is formed in a substrateto be cut, in accordance with rotation of the scribing wheel 1. Thecrack-formed substrate is then fed to a breaking device, in which apressure is applied to the substrate along the crack by a breaking bar,thereby cutting the substrate.

FIGS. 4 and 5 are schematic views respectively illustrating scribing andbreaking processes involved in the related art substrate cutting method.In the scribing process, the scribing wheel or cutting wheel 82 bringsinto contact with the surface of a substrate 81, as shown in FIG. 4. Insuch a state, the scribing wheel 82 is rotated along the substrate 81while applying pressure of about 2.40 Kgf/cm² to the substrate 81. As aresult, a crack 83 with a certain depth is formed in the surface of thesubstrate 81 along a track of the scribing wheel 82.

Thereafter, the breaking process is carried out along the crack 83 inthe surface of the substrate 81 to cut the substrate 81. That is, asshown in FIG. 5, a breaking bar 84 is arranged on the substrate 81 inwhich the crack 83 has been formed in the scribing process. The portionof the breaking bar 84 coming into direct contact with the surface ofthe substrate 81, that is, a portion A of the breaking bar 84, is madeof a material which is sufficiently hard, but does not form scratches onthe surface of substrate 81, such as urethane rubber.

Next, pressure is momentarily applied to the substrate 81 by thebreaking bar 84 under the condition in which the breaking bar 84 isaccurately aligned with the crack 83. As a result, the crack 83 isextended, thereby causing the substrate 81 to be cut.

Thereafter, a grinding process is carried out using a grindstone havinga predetermined mesh size, in order to grind cut surfaces and corners ofthe substrate formed after the scribing and breaking processes.

Thus, in accordance with the related art substrate cutting method, aplurality of liquid crystal panels formed on the substrate are separatedinto a plurality of unit LCD devices in accordance with theabove-mentioned scribing and breaking processes. However, theabove-mentioned related art substrate cutting method has variousproblems. For example, the scribing wheel used in the scribing processfor cutting the substrate is expensive and has a short lifespan,necessitating periodic replacement thereof. For this reason, an increasein manufacturing cost is incurred.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an apparatus forcutting a substrate and a method using the same that substantiallyobviate one or more problems due to limitations and disadvantages of therelated art.

An object of the present invention is to provide an apparatus forcutting a substrate which uses condenser lenses having different depthsof focus in a process of cutting a substrate using a femtosecond laser,thereby reducing a taper angle of the cut surface.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, anapparatus for cutting a substrate includes a laser oscillator generatinga femtosecond laser beam, a first beam splitter splitting thefemtosecond laser beam into first and second femtosecond laser beams, afirst condenser lens receiving the first femtosecond laser beam andcondensing the first femtosecond laser beam to have a first focal depth,a second condenser lens receiving the second femtosecond laser beam, andcondensing the second femtosecond laser beam to have a second focaldepth different from the first focal depth, and a second beam splitterreceiving and splitting the first femtosecond laser beam condensedthrough the first condenser lens and the second femtosecond laser beamcondensed through the second condenser lens, and irradiating the splitfirst and second femtosecond laser beams at different positions on asubstrate to be cut.

In another aspect of the present invention, an apparatus for cutting asubstrate includes a plurality of laser oscillators generating at leastfirst and second femtosecond laser beams, a plurality of condenserlenses receiving the at least first and second femtosecond laser beams,and condensing the received at least first and second femtosecond laserbeams to have first and second focal depths being different from eachother, respectively, a reflector reflecting the first condensedfemtosecond laser beam, and a beam splitter receiving and splitting thecondensed at least first and second femtosecond laser beams, andirradiating the split at least first and second femtosecond laser beamsat different positions on a substrate to be cut.

In a further aspect of the present invention, a method for cutting asubstrate includes arranging the substrate on a stage and generating afemtosecond laser beam from a femtosecond laser oscillator, splittingthe femtosecond laser beam into at least first and second femtosecondlaser beams and condensing the at least first and second femtosecondlaser beams to have different focal depths, and irradiating thecondensed at least first and second femtosecond laser beams onto thesubstrate at different positions, thereby cutting the substrate.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a cross-sectional view illustrating the related art LCD;

FIG. 2 is a flow chart illustrating the related art LCD manufacturingmethod;

FIG. 3 is a schematic view illustrating the related art scribing device;

FIGS. 4 and 5 are schematic views respectively illustrating scribing andbreaking processes involved in the related art substrate cutting method;

FIG. 6 is a schematic view explaining a method for cutting a substrateusing a femtosecond laser in accordance with an embodiment of thepresent invention;

FIG. 7 is an enlarged view corresponding to a portion “A” of FIG. 6,showing a cross-sectional view of the substrate after the substratecutting process is carried out using the femtosecond lasers havingdifferent depths of focus in accordance with an embodiment of thepresent invention; and

FIG. 8 is a schematic view illustrating a femtosecond laser generatingapparatus according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

Generally, ablation carried out by a laser is mainly used for themanufacture of high-accuracy precision elements. Where high-speed laserpulses are used, there is an advantage in that it is possible to reducedamage of the substrate around a region where the laser pulses areirradiated. For this reason, laser machines using a YAG laser or excimerlayer having a pulse speed on the order of nanoseconds, for example,10⁻⁹ m/s, are generally used. Such a laser machine is called a“nanosecond laser machine”.

However, YAG laser machines, in which aluminum oxide is artificiallycrystallized to generate a laser, have a problem in that side wallsmachined by a YAG laser tend to be rough. In the case of a CO₂ laser,which is an infrared-based laser, there is a drawback in that cratersmay be formed at the machined region. For this reason, such laserscannot be used for a micro machining process requiring an accuracy tothe micrometers of higher.

That is, the above-described machining may be referred to as a “thermallaser machining”, because the machining is carried out using thermalenergy converted from optical energy. For this reason, using such amachining method, it is difficult to achieve precise machining becausethe machined structure may be easily collapsed.

On the other hand, in the case of an excimer laser, sublimate etching iscarried out in accordance with an opto-chemical reaction causingdisconnection of covalent bonds of carbon atoms. Accordingly, precisemachining is possible. That is, when an excimer laser is irradiated tothe surface of an object to be machined, the irradiated surface portionof the object is dissipated together with plasma and shock noise, andthus, is decomposed. Thus, ablative photo decomposition is carried out,so that endothermic evaporation machining is achieved which can providea high machining accuracy.

The energy of the excimer laser is not used only for the disconnectionof the covalent bonds of carbon atoms. In other words, a part of theexcimer laser energy is converted to thermal energy. Since the excimerlaser energy has a high density, the effect of the converted thermalenergy is considerable. For this reason, it is difficult to machinemineral materials such as metal, ceramic, and silicon, and quartz orglass having a low photo absorption rate, using the excimer laser. Athermal deformation generated in this case adversely affects durabilityof the machined product, even through the thermal deformation is causedby thermal energy lower than that of the latter case.

On the other hand, the femtosecond laser, the pulse speed of which isabout 10⁻¹⁵ m/s, has superior characteristics for solving theabove-described problems. This is possible because a very highoscillation density of laser energy can be obtained when a laseroscillating for an ultra-short pulse radiation duration of 1 picosecondor shorter, that is, 1×10⁻¹² m/s or shorter, is used. When a laser hasphoto energy of 1 mJ, and a pulse radiation duration of 100 femtosecondsor shorter, the energy density of the laser reaches a level of about 10Gigawatts. In this case, accordingly, it is possible to machine anymaterials.

Meanwhile, when an ultra-short pulse laser such as a femtosecond laseris radiated to an object to be machined, a multiphoton phenomenon occursin the lattices of the material of the object, thereby causing the atomsof the material to be excited. However, the duration of the incidentlaser pulses is shorter than the time taken for photons to transfer heatto the lattices around the photons during the excitation of the atoms.Accordingly, it is possible to solve problems of a degradation inmachining accuracy, physical and chemical variations in the propertiesof the material, and a partial melting of the machined portion of theobject, which are caused by heat diffusion occurring during themachining of the object. Thus, high-precision machining can beperformed.

Moreover, accumulation of particles does not occur during thefemtosecond laser machining. Also, little or no by-products, such ascraters, are formed. Accordingly, it is unnecessary to use a by-productremoving process such as an ultrasonic cleaning process, which isrequired in the related art methods. It is also possible to machine amaterial having a high heat transfer coefficient or a low photoabsorption rate. Further, it is possible to machine two or moredifferent materials, or a composite material having a multi-layerstructure by using a single process.

Hereinafter, a femtosecond laser generating apparatus according to anembodiment of the present invention and a method for cutting a substrateusing the femtosecond laser generating apparatus will be described withreference to the annexed drawings.

FIG. 6 is a schematic view illustrating a femtosecond laser generatingapparatus according to an embodiment of the present invention. As shownin FIG. 6, the femtosecond laser generating apparatus includes afemtosecond laser oscillator 200 for generating a femtosecond laser beam201, and a first beam splitter 210 for splitting the femtosecond laserbeam 201 generated from the femtosecond laser oscillator 200 into afirst femtosecond laser beam 201 a and a second femtosecond laser beam201 b. The femtosecond laser generating apparatus also includes a firstmirror 220 for reflecting the first femtosecond laser beam 201 aemerging from the first beam splitter 210, a first condenser lens 230for receiving the first femtosecond laser beam 201 a reflected by thefirst mirror 220 and condensing the first femtosecond laser beam 201 asuch that the first femtosecond laser beam 201 a has a depth of focusf1, and a second condenser lens 250 for receiving the second femtosecondlaser beam 201 b emerging from the first beam splitter 210 andcondensing the second femtosecond laser beam 201 b such that the secondfemtosecond laser 201 b has a depth of focus f2 different from the depthof focus f1.

The femtosecond laser generating apparatus includes a second mirror 240for reflecting the first femtosecond laser beam 201 a condensed throughthe first condenser lens 230, and a second beam splitter 260 forreceiving and splitting the first femtosecond laser beam 201 a reflectedby the second mirror 240 and the second femtosecond laser beam 201 bcondensed through the second condenser lens 250, and irradiating thesplit first and second femtosecond laser beams 201 a and 201 b to thesubstrate 100 at different depths, respectively.

Although the oscillating femtosecond laser beam 201 generated from thefemtosecond laser oscillator 200 is split into two femtosecond laserbeams, that is, the first and second femtosecond laser beams 201 a and201 b in the illustrated case, the femtosecond laser beam 201 may besplit into three or more femtosecond laser beams which may be, in turn,irradiated to different positions of the substrate to be cut, using aplurality of condenser lenses having different depths of focus,respectively.

In addition, an optical attenuator (not shown) may be arranged betweenthe first mirror 220 and the first condenser lens 230, in order toadjust the energy of the femtosecond laser beam in accordance with thekind of the substrate to be cut (e.g., material and thickness of thesubstrate). The optical attenuator may be of a disc type wherein anattenuation operation is achieved in accordance with the rotation of theoptical attenuator. In this case, however, the beam exhibits differentattenuation levels at different portions of the cross-section thereof,when the beam has a large diameter. Accordingly, it is preferred thatthe optical attenuator include a λ/2 plate rotatable in accordance withdriving of a motor, a linear polarizer, and a λ/4 plate, in order toattenuate the intensity of the laser beam by 99% at maximum.

Each of the first and second mirrors 220 and 240 may be formed with acoating adapted to reflect a large part of the femtosecond laser beam ata predetermined angle while transmitting the remaining part of thefemtosecond laser beam therethrough such that the transmittedfemtosecond laser beam passes through a photodiode (not shown) adaptedto measure the energy of the femtosecond laser beam. In this case, theoptical attenuator can be controlled in accordance with the measuredintensity of the femtosecond laser beam, to achieve an enhancement inaccuracy.

Meanwhile, when the femtosecond laser beam 201 is generated from thefemtosecond laser oscillator 200, it may be possible to identify acutting position on the substrate 100 using a CCD camera (not shown)arranged on the same axis as the generated femtosecond laser beam 201.Also, an image of the substrate 100 may be displayed through a displaydevice 280, in order to accurately cut a desired portion of thesubstrate 100.

Since the femtosecond laser, which is used in accordance with anembodiment of the present invention to cut a mother substrate formedwith a plurality of liquid crystal panels, for separating the mothersubstrate into a plurality of unit liquid crystal panels, has a shortpulse width (about 150 fs) and a high peak power per pulse, thermalexpansion and generation of shock waves do not occur around a portion ofthe substrate 100, which is cut, during the cutting operation.

Meanwhile, the femtosecond laser beam has characteristics different fromthose of general laser beams. That is, “monochromaticity”, which is oneof the laser characteristics, does not exist in the femtosecond laserbeam. Contrary to general lasers, the femtosecond laser has aconsiderably wide spectrum range. Also, the femtosecond laser beamamplified through the condenser lenses having different depths of focushas a peak power much higher than those of general laser beams, forexample, a peak power on the order of terawatts (10¹² watts). Recently,such an amplified femtosecond laser has exhibited a peak power increasedto petawatts (10¹⁵ watts).

Accordingly, the femtosecond laser may be called “T3 laser (Table TopTerawatt Laser)”. It is possible to greatly increase the density of thelaser by simply condensing the laser through a condenser lens. When thelaser beam is focused onto an object, the material of the object aroundthe focus is indeed instantly changed to a plasma state.

With some exceptions, the femtosecond laser generally has a pulse energyon the order of micro-Joules (μJ) per pulse. In some cases, thefemtosecond laser has a stronger pulse energy on the order ofmilli-Joules per pulse corresponding to mean power of about 1 Watt.

Plasma generated by a general laser reacts with the laser to absorb thelaser or to heat the material to be machined. As a result, such plasmacauses various problems such as an increase in heat affect, unstablemachining, and a degradation in efficiency. However, the femtosecondlaser changes such circumstances caused by plasma.

Generally, an acceptor receiving the energy of a laser at the side ofthe material to be machined by the laser is an electron. In the case ofa metal, the acceptor has a free electron existing in a conduction bandor an electron excited into the conduction band by light. The electron(electron system) is vibrated by a vibrating electric field of thelaser. That is, the electron receives energy from the laser. Thevibrating electron strikes atoms or ions existing in the lattices of thematerial (lattice system), thereby providing atoms or ions with akinetic energy to (i.e., causing an increase in the temperature of thematerial). As a result, the phase of the material is changed (melting orevaporation), thereby causing the material to be machined.

The time taken for the energy to be transferred from the electron systemto the lattice system is on the order of femtoseconds. Accordingly, inthe femtosecond laser machining, absorption of the laser energy andchange of the material (to be machined) following the laser energyabsorption are temporally separated from each other.

For example, the time taken for the atoms of the irradiated material tobe ionized, and thus, to generate plasma, is longer than the pulse widthof the femtosecond laser. Accordingly, it is expected that the plasmacannot react with the laser. Furthermore, the time taken for the heatgenerated at the irradiated region to be diffused around the irradiatedregion is shorter than the pulse width of the femtosecond layer. Theenergy of the laser exists locally in the irradiated region, so that thephase change of the material occurs only in the irradiated region. Thus,when a substrate is cut by using the femtosecond laser in accordancewith an embodiment of the present invention, the cutting is achievedwithout formation of a heat affected zone around the region where thecutting is carried out.

Hereinafter, the substrate cutting method using the femtosecond lasergenerated from the femtosecond laser generating apparatus according toan embodiment of the present invention will be described in more detail.

In accordance with this substrate cutting method, a substrate 100, whichis a mother substrate formed with a plurality of liquid crystal panels,and is to be cut for separating the mother substrate into a plurality ofunit liquid crystal panels, is first arranged on a movable stage (notshown). Thereafter, a femtosecond laser beam 201 is generated from thefemtosecond laser oscillator 200.

Meanwhile, when the femtosecond laser beam 201 is generated from thefemtosecond laser oscillator 200, identification of a cutting positionon the substrate 100 is carried out using the CCD camera (not shown)arranged on the same axis as the generated femtosecond laser beam 201.Also, an image of the substrate 100 is displayed in order to accuratelycut a desired portion of the substrate 100.

Subsequently, the intensity and density of the femtosecond laser beam201 generated from the femtosecond laser oscillator 200 are adjusted.The adjusted femtosecond laser beam 201 is then split through the firstbeam splitter 210 into a first femtosecond laser beam 201 a and a secondfemtosecond laser beam 201 b. The femtosecond laser beam 201 a emergingfrom the first beam splitter 210 is reflected by the first mirror 220.

The first femtosecond laser beam 201 a reflected by the first mirror 220is condensed through the first condenser lens 230 which has the firstdepth of focus f1. The first femtosecond laser beam 201 a condensedthrough the first condenser lens 230 having the first depth of focus f1is then reflected by the second mirror 240.

Subsequently, the second femtosecond laser beam 201 b emerging from thefirst beam splitter 210 is condensed through the second condenser lens250 which has the second depth of focus f2 different from the depth offocus f1. The second beam splitter 260 receives and splits the firstfemtosecond laser beam 201 a reflected by the second mirror 240 and thesecond femtosecond laser 201 b condensed through the second condenserlens 250, and irradiates the split first and second femtosecond laserbeams 201 a and 201 b to the substrate 100, to be cut, at differentpositions. In this state, the substrate 100 is cut while moving thestage in one direction.

The cutting of the substrate 100 may be carried out while moving thefemtosecond laser oscillator 200 in one direction in a fixed state ofthe stage. In order to allow the operator to check the cutting conditionduring the cutting process, a corresponding image picked up by the CCDcamera 270 may be displayed through the display device 280 as amonitoring device.

FIG. 7 is an enlarged view corresponding to a portion “A” of FIG. 6,showing the cut state of the substrate after the substrate cuttingprocess is carried out using the femtosecond lasers having differentdepths of focus in accordance with an embodiment of the presentinvention.

When it is desired to cut the substrate 100, which is a mother substrateformed with a plurality of liquid crystal panels, for separating themother substrate into the unit liquid crystal panels, femtosecond laserbeams having different depths of focus f1 and f2 are irradiated to thesubstrate 100 at different positions f1′ and f2′, respectively, forcutting of the substrate 100, as shown in FIG. 7.

When the cutting process is carried out at a cutting width of about 40μm, neither thermal expansion nor generation of shock waves occur aroundthe region where the cutting is carried out. Under this condition,accordingly, the substrate 100 is uniformly and accurately cut in adesired cutting direction.

That is, when the substrate is cut by irradiating a plurality offemtosecond laser beams having different depths of focus at differentpositions, it is possible to minimize the angle of a taper surfaceformed at a region where the substrate is cut. Accordingly, it ispossible to accurately and cleanly cut the substrate without formingpaddings or depositions on the edge and side surfaces of the cutsubstrate portion.

FIG. 8 is a schematic view illustrating a femtosecond laser generatingapparatus according to another embodiment of the present invention. Asshown in FIG. 8, the femtosecond laser generating apparatus includes afirst femtosecond laser oscillator 300 and a second femtosecond laseroscillator 400 which generate femtosecond laser beams 301 and 401,respectively, and a first condenser lens 310 and a second condenser lens410 which receive the first and second femtosecond laser beams 301 fromthe first and second femtosecond laser oscillator 300 and 400 andcondense the first and second femtosecond laser beams 301 and 401 suchthat the first and second femtosecond laser beams 301 and 401 havedifferent depths of focus f1 and f2, respectively.

The femotosecond laser generating apparatus also includes a mirror 320for reflecting the first femtosecond laser beam 301 condensed throughthe first condenser lens 310, and a beam splitter 420 for receiving andsplitting the first femtosecond laser beam 301 reflected by the mirror320 and the second femtosecond laser beam 401 condensed through thesecond condenser lens 410, and irradiating the split first and secondfemtosecond laser beams 301 and 401 to a substrate 500 at differentpositions.

Although two femtosecond laser oscillators 300 and 400 are used in thiscase, three or more femtosecond laser oscillators can be used togenerate three or more femtosecond laser beams having different depthsof focus, for cutting of a substrate. Further, the number of mirrorsused for three or more femtosecond laser oscillators having differentdepths of focus may be one less than the number of the femtosecond laseroscillators.

As apparent from the above description, the femtosecond laser generatingapparatus according to embodiments of the present invention and thesubstrate cutting method using the same have various effects. Forexample, since a plurality of femtosecond lasers having different depthsof focus are irradiated to a substrate, to be cut, at differentpositions, respectively, for cutting of the substrate, it is possible tominimize the angle of a taper surface formed after the cutting.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the apparatus for cutting asubstrate and the method using the same of the present invention withoutdeparting from the spirit or scope of the inventions. Thus, it isintended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An apparatus for cutting a substrate, comprising: a laser oscillatorgenerating a femtosecond laser beam; a first beam splitter splitting thefemtosecond laser beam into first and second femtosecond laser beams; afirst condenser lens receiving the first femtosecond laser beam andcondensing the first femtosecond laser beam to have a first focal depth;a second condenser lens receiving the second femtosecond laser beam, andcondensing the second femtosecond laser beam to have a second focaldepth different from the first focal depth; an optical attenuatorbetween the first reflector and the first condenser lens to adjustenergy of the femtosecond laser beam; and a second beam splitterreceiving and splitting the first femtosecond laser beam condensedthrough the first condenser lens and the second femtosecond laser beamcondensed through the second condenser lens, and irradiating the splitfirst and second femtosecond laser beams at different depths in thedirection vertical to the substrate to be cut, wherein the opticalattenuator includes a λ/2 plate, a linear polarizer, and a λ/4.
 2. Theapparatus according to claim 1, further comprising: a first reflectorreflecting the first femtosecond laser beam emerging from the first beamsplitter to the first condenser lens; and a second reflector reflectingthe first femtosecond laser beam condensed through the first condenserlens to the second beam splitter.
 3. The apparatus according to claim 1,wherein the optical attenuator includes a disc type.
 4. The apparatusaccording to claim 1, further comprising: a charge coupled device cameramonitoring an image of cut portions of the substrate; and a displaydevice displaying the image monitored by the charge coupled devicecamera.
 5. The apparatus according to claim 1, wherein the substrate isa mother substrate on which a plurality of liquid crystal panels areformed.
 6. A method for cutting a substrate, comprising the steps of:arranging the substrate on a stage and generating a femtosecond laserbeam from a femtosecond laser oscillator; splitting the femtosecondlaser beam into at least first and second femtosecond laser beams andcondensing the at least first and second femtosecond laser beams to havedifferent focal depths; adjusting an intensity and a density of thefemtosecond laser beam generated from the femtosecond laser oscillatorusing an optical attenuator; and irradiating the condensed at leastfirst and second femtosecond laser beams onto the substrate at differentpositions, thereby cutting the substrate, wherein the optical attenuatorincludes a λ/2 plate, a linear polarizer, and a λ/4 plate are the threeparts.
 7. The method according to claim 6, wherein the stage is amovable stage.
 8. The method according to claim 6, further comprisingidentifying cut portion of the substrate corresponding to the at leastfirst and second femtosecond laser beams-irradiated positions by a CCDcamera.
 9. The method according to claim 6, wherein the cutting of thesubstrate is carried out in accordance with a movement of the stage inone direction in a fixed state of the femtosecond laser oscillator. 10.The method according to claim 6, wherein the cutting of the substrate iscarried out in accordance with a movement of the femtosecond laseroscillator in one direction in a fixed state of the stage.
 11. Themethod according to claim 6, further comprising monitoring a cuttingcondition when the substrate is cut, so that an operator can identifythe cutting condition.